Audio Amp / Crossover Design

I would have thought that you want the crossover after the amp otherwise you will be forever trying to balance the levels.

Low-pass, band-pass and high-pass active filters feeding separate bass, midrange, and tweeter amplifiers would give you complete control over each band. A simple passive crossover rolls off too slowly. Active filters can give you much sharper roll-off and band definition, depending on design. Using VCAs (Burr-Brown makes some very nice VCAs) in your active-filter design allows for easy tuning, if that is desired. You might try starting with a state-variable AF using VCAs and play with it. Or if you prefer separate AFs, elliptical filters give you fastest roll-off (in exchange for a bit of pass- and stop-band ripple).

Just some ideas.

Having the crossover filters before the power amplifiers is the best condition...

The filters as errrm the last poster said can be easily modified at the low power levels and use better / cheaper components...

Also, if you feed all the signals through one amplifier the distortion in that amplifier will cause intermodulation products with the frequencies which would have been filtered out if the filtering was before the amplifiers, so the output should be a more faithful and distortion free signal - all things being equal...

The downside is of course that you will need three power amplifiers for each of the bands, although this could be countered by saying at high frequencies (tweeter drive) much less power is required than for the (bass) low frequency amplifier...

I'm not an audiophile and laugh when I see people buying gold plated mains fuses on ebay for £20 each, because the sound is so much crisper or whatever!!!

John.

Here's one for you.

What you seem to be asking for is an active crossover network, rather than a passive one.

An active network is placed between the signal source and amplification, and requires a power connection. A really good benefit of an active network is that you can use lower power amplification (35-50 watts per channel) rather than high power amps (500+ watts per channel) and get a much cleaner sound.

A passive network is most commonly nothing more than a few capacitors connected to the speaker coils. Highly inefficient, and as stated previously, not a great amount of separation.

Use your favorite search engine with the term active+crossover (I came up with 2.3 million sites) some with very good explanations, and some with actual plans for constructing your own.

Have fun with this one!

C

Thanks, C! I went to the link you posted and it's exactly what i'm looking for.

Phoenyx writes: "I'm building an audio amplifier from scratch for my truck..."

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Yes, C's post links to a nice design. As you're building this crossover for your truck, you'll be deriving the power from your truck's 12v system, and so you can omit the power transformer and bridge rectifier sections (leave the filter caps in the circuit). As a precaution against ignition noise, you might wish to insert a low-value power resistor (say, 1-5 ohms) in the positive lead ahead of the caps. Alternatively, LC power filters are available at nominal cost. Do build your crossover in a metal enclosure and ground the enclosure to your truck's body/chassis/frame.

why are you building it?

Cheaper? no

Control over design? yes, but with so many to choose from why bother.

Ultimate sound quality? If you are asking questions like where to put crossover, leave it to someone who has been doing it awhile.

I can point you in the right direction for the proper equipment to get the job done, if you truly want sound Q. I have worked with the best in the business building winning show cars.

Fun of doing it yourself? Right on!!!!

If the latter, I can help with powersupply design, outputstage design, and heatsinking tips. ( I work in the mobile electronic industry-been to many of the manufacturing facilities in US and built products myself)

I used to work for a sound contractor once. I found it curious that even in such a company, not all the engineers agreed on these things.

The purists preferred the active crossovers (crossover before the amps) since (as mentioned already) you can adjust the levels so that they're perfect.

The theorists say that you should design your passive, crossover filters (crossover after the amps) so that the frequencies are balanced in level and distribution depending on the drivers you're using.

Our boss was a real genius, however. He took the purists and theorists to one of our testing rooms where he set up a complicated system where he could switch between active and passive crossover through the same speakers. Then he played test tones, voice announcements, and music through the system, switching between the two configurations. He challenged them to identify when active crossover or passive crossover was being used.

Everyone was amazed because no one could really tell with certainty. Our boss declared that good sound was in the ears of the listeners. You can tweek any system to provide good sound.

He also once played a CD player against a good quality tape deck and, again, no one could tell the difference. He used the same speakers and amplifiers for both.

Just curious, why are you putting a rock concert style amp set in a TRUCK? How you gonna filter out the engine noise, the tire noise, rain on the roof, slapping windshield wipers, the police sirens, that impossible to find rattle in the dashboard, and the absurdly small space with weird sound reflecting surfaces all over it? The best equalizer in the world won't do better than "adequate".

Rules of thumb...more power does NOT equal better frequency responce (just the opposite in fact), put your money into pre-amps, use horns as much as possible, and impedance match your speakers with your power amps.

Buy good quality components, not necessarily the most expensive, and make sure they are well matched. A room mate of mine blew his 200 dollar power amps when he mounted an extra couple of speakers in parallel to give himself "surround sound". Don't be that guy!

Last time I bought vehicle speakers, the cross over network was built into the speaker set. Sound came from a tape deck, went to a pair of pre-amps, then a fairly simple equalizer, and then a couple of power amps. People complimented me on the sound. This being the new millenium, and amps are now so cheap, I can see why you would take the signal, split it with a cross over network, and send it into three pre-amps per side, same is if you were setting up the amps for a rock band. To my mind though, that gives three times the opportunity to screw up the signal. What would screw up the signal? Well, stray emf (static) would do it...static discharge from tires, overhead power lines...you know, environment!

(An ipod would give better sound in my humble opinion.)

Putting a high end audio set up has nothing to do with logic or sense as I found out when my son built a system in an Austin Metro van. If you don't know the Metro van it's based on the car that was the last variant of the Austin mini so imagine a mini with the rear paneled in. My son built a 3kW stereo system in there with the whole of the load space being used as a bass box for a single 18" speaker, the other speakers were in the front doors. I was impressed by the engineering but had to ask why???

I see that you are in Orlando, If you want to purchase equipment or get real advice on making your truck sound incredible, let me know. It was my territory when I worked with JBL, Infinity, Diamond Audio, and many others. I know every shop and most of the good installers in town. If you really want to do some cool digging through some older but good quality equipment I can suggest a cool hole in the wall shop with more inventory than anybody.